The instant invention relates generally to an instrument for measuring the pH level of fluids of high consistencies, slurries, and the like, passing through a pressurized process pipe. More particularly, the instant invention relates to mechanisms for protecting the instrument from harm attributable to the material being conveyed by the process pipe and for introducing the instrument into the process pipe.
The paper industry has utilized diverse probe-like instruments for measuring the pH of the pulp stock in pulp and paper mills. The pH level, which is a measure of the acidity or alkilinity of the product, is effectively determined by a pH probe. Most known pH probes employ a glass electrode and a reference electrode, generally retained within the same housing and identified as a "combination" electrode. The glass electrode includes a pH sensitive glass membrane attached to a glass tube containing a metal electrode and a saturated aqueous solution, such as potassium chloride, or a metal-metal salt, such as silver-silver chloride. The reference electrode is retained within a space filled with the same type of solution and has a porous wall. The electrodes function to generate an electrical signal which is proportional to the pH level in the fluid being measured. The electrical signal, when appropriately amplified, may be used to adjust operating parameters upstream or downstream of the probe.
Representative pH probes are disclosed in U.S. Pat. No. 4,112,352, granted to T R. Barben II, U.S. Pat. No. 4,008,141, granted to H. Kotani et al, and U.S. Pat. Nos. 4,202,749 and 4,383,908, both granted to T. C. Phelps et al.
Known pH probes functioned satisfactorily under low pressure flow conditions when the fluid is of low consistency (percent solid matter), or when employed to measure the pH in an off-line or batch process. However, known pH probes were found to be susceptible to breakage and clogging, and short service life, when exposed to harsh, on-line, continuous operating conditions, such as those encountered in pulp and paper mills, chemical processing plants, or the like. The component found to be most prone to failure proved to be the glass sensing electrode which projects into the stream flowing through the process pipe.
The glass pH sensing electrode 10 shown in FIG. 2 of the Barben, II patent projects beyond the rigid cylindrical housing that contains the other components of the instrument. A series of toroidal hardwood plugs 20 fit within the cylindrical housing and serve as the porous junctions of the reference electrode. The end of the electrode 10, however, is unprotected and unsupported, and has been easily broken by solid objects that may normally, or inadvertently, be in the flowing process stream.
Kotani et al recognizes the fragile nature of glass sensing bulb 5 which includes a glass membrane 8, and provides a pH probe that allows the sensing bulb to be easily replaced. A side wall 17 of a strong, yet porous, material is utilized, as noted in column 2, lines 24-35 of such patent, to surround a major portion of the body of the probe. A cap 14 with a central bore receives electrode 5, and the projecting flanges surround the glass membrane.
However, under field conditions and particularly when measuring pH values in a stream with high solids concentration, or high consistency, such as wood pulp slurry, the pulp can accumulate into a solid mass between the flanges and sensing bulb and eventually will interfere with the accurate determination of pH. Thus the pH probe must be removed from the process stream frequently to permit cleaning or electrode replacement which is a costly and counterproductive procedure, particularly when the high levels of paper mill production dictate continuous, on-line pH monitoring, whenever possible.
Phelps et al '749 and Phelps '908 disclose a pH probe system that may be removably installed in-line in a pressurized pipe line to determine the pH of the fluid flowing therethrough; see FIG. 1 in each patent. The probe is coated with a smooth adherent coating 37, such as woven filaments of glass impregnated with a plastic resin and a hardener. The coating defines an elongated sheath with a uniform, round circumference. A sensor head with a glass electrode tip 38A is shown in FIG. 2 of each patent. A protector shield 41 fits over the sensor head and includes a flange 42 positioned in the direction of the flow in order to protect the sensor portion of the probe.
The protector shield shown in the Phelps et al patents, however, requires the shield for the pH probe system be properly oriented upon insertion into the process flow line, and this orientation may not be readily attained. Additionally, during purging or draining or other conditions leading to reverse flows within the process pipe, the flange will be ineffective and the sensing electrode will be subjected to reversely directed forces, or impacted by solid objects, which may break or otherwise damage the sensing electrode. Furthermore, the flange on the protector shield may cause vortex shedding or other flow disturbances, such as stagnation of flow on the downstream side of the flange. Such disturbances contribute to the accumulation of solids or pulp between the flange and the sensing electrode. This may cause formation of a dewatered mass that may eventually cover part, or all, of the sensing electrode so that the pH measurement will not accurately reflect the actual pH of the process stream.